Pulse repeating system



Jann 9, I s. I. PULSE REPEATING SYSTEM Filed July' 1o, 19,45

. @ML/Nc PuLsEs FRoM cALL/Nc Wo @5R/NG DISCOw/VNE CT SEIN/RE ANSWER/NG SWITCH H001( FL .4S/'IES man cAL/.Eo E/vb /NVENTOR By 5J. COPY FIG. 3 H-susw-'LA sHEs-v- FROMl CALLED END ATTORNEY Patented Jan. 9, 1945 i, N UNITED f STATE s? N PATENT OFFICE l PULSE REPEATING SYSTEM t t Samuel I. Cory, Towaco, `N. J.,.assignor to Bell Telephone Laboratories, Incorporated, New Yor'k,iN. Y., a corporation of New `York i Application July 1o, 1943-scria1`No- 194,198" t claims (01.179s4y t The invention relates to pulse` repeating sys-` tems I and more particularly to systems `of this" type operating on a duplex basis.

outgoing signals when the impedance balance is perfect. For this purpose the polar relay may It is an object ofthe invention to provide a a pulse 'repeating system witlifa simplied duplexl balance and capable of efficient operation under conditions of` line `variations and interference.

It is a `more speciiic object of the invention to` provide a system of thistype whichwith standardized equipmentis adapted for repeating selecting `and 'supervising pulses for telephone switching systems` Over lines of characteristics and lengths.

The selective and supervisory operationsof au: tornatic` `telephone switching systems are fre- `quently performed over extensivealine circuitsg,

such as toll lines, the lengths 4ntf, which differ greatly and which are subject to well-known disturbances i and` interferences. The" selecting pulsesjusually `termed the dial pulses, `are usually transmittedwfrom one telephone exchange to another` over half-duplex pulse repeating circuits,

which hitherto have "been carefully balanced td operate satisfactorily under the changing `operating conditions inthe line circuit. The reason for duplexing the `p`ulse`circuit" is mainly tobe found in the fact that thetrafc requirements ini t clude the transmission inboth directions over thel be connected in any well-known manner, as by o having its operating windings directly included in the two balanced circuits or` connected in a bridgebetween the balanced circuits, in accordiance with well-known practiceintelegraph reg peating systems. `The system is also equipped for the transmission of busy lsignals comprising a seg greatly` diiering R ries of pulses of a predetermined frequency,` usually `about 80M cycles per minute, which are impressed` upon the' duplex circuit; by operation of the pole changer. i

As is `wellknown it has hithertobeen necessary to establish a fair balance between [the `line cir-,i`

cuit andthe balancing network induplex sys-` tems of either type referredto above,` in order toprevent thejreceivingrelayifrom responding to `theoutgoing pulses@ When signals: are impressed onan unbalanced system unbalance currents arise which may operate the receivingrelay.` Thus-in the diierential duplex circuiti the4 i currentsin the two operating windings willbe unequal or unbalanced and the stronger current may operate the relay. In the bridge duplex` circuit i `the potentials applied to the relay winding are line circuit of supervisory signals orpulses." The` operations are complicated by the requirements be termed the; idle and the busy` conditions.

The pulse repeating system is `adaptedfto re- 2 30 that, in the series of operations involved ing 'theffL setting up of` a connection, the linetcircuitis" placed under two dinerent conditionswhich may t more arranged softhat it will be unecessary to ceve dialing pulses` froma calling station,;whic h may be a` remote telephonesubscribers` stationl` or a local operatorsdialing` circuit, and to repeat the pulses over'yan `outgoing line circuit to,` a

distant telephone exchange, equipped for autof matic switchingjin response to the dial pulses 'for scribers station.` i f 4 i 'Therepeating circuit includes a pole changing relay for applying twodiflerent transmitting potentials to the'line circuit corresponding tothe two conditions of the pulses,pnamely, marking` the selection of another exchange or anothersub`` and spacing or current` and no current., i `The pulses from the pole changing relay are impressed `upon a duplex circuit including thel line circuit in one branch and the -`iltfalancingf*` circuit the other branch.

`Apelar receiving relay is included` infine uupnex` circuit for reception of` signals incoming over" the line circuit, thisrelay being unaiected i unequal and an equalizer or imbalance current i the windings and may operateY i will ow through the relay. i i In accordance with a feature of `the invention,`

the balancing network is simplified and is stand..`

adized to supplya fair balance for a large variety of linecircuits so that it may be manufactured at comparatively lowcost. In accordance with the invention the system is further` adjust the balancing Vnetwork to compensatefor i sporadic variations in the line circuits during ordinary operating conditions, including `temperature and leakage changes. This` simplification of the balancing network is further effective in vreducing the large amount of designing work in` volved in the provision of a large variety `of differently dimensioned balancing networks for the' corresponding varieties fof `line circuits to satisfy the `present daysevere demands for accuracy.`

For the purposes justioutlnedthe invention provides an arrangement for preventing false opt. erations, whichwouldbe due to improper re-V sponses by the receiving relay during the trans-1L i missionof outgoing pulses resulting fromfun-` balanced conditions. i, By this provision a given` `design of balancing network` may-` be `based on vcertain average `or `predominant line conditions Aamong the large variety ofconditions met with 'switchhook iiashesmay be transmitted in either i ineffective during the transmission of outgoing in the plant. Such a network may thus provide an accurate balance for a large number of lines with average characteristics it would however be less accurate in the balancing of line circuits having more diverging characteristics, without, however, establishing a circuit condition which would result in false operations due to unbalance conditions caused by line variations or interference.

When the pulse repeating system is idle, the-- pole changing or sending relay will apply an idle `10 condition to the toll line, which may be assumed to be a Spacing orno `current condition, theother end of the line circuit being assumed to` apply a similar condition to the line. In response to the seizure of the repeater station by a -calling station or an operator, the pole changer applies` a busy or marking condition to the toll line. After the transmission of dialing lpulses, the distant station also applies a marking or answering condition to the line circuit which consequently f will. operate .thev .polar receiving relay at `,they repeatingstation. y

After this condition. has been reached.. further v pulses. mayV be transmitted in either directionover the. line circuit. Forexample, a rering pulse mayibe. transmittedfby'thesending station in= theorm. of apulsewhich willl cause thedistant eXchange-.toreringmhe called line; busy ilashes may be transmittedbackto the callingline; and

direction. n

The polar, receiving ,relay at .the. .repeater station'willV respond to incomingpulses, from.. the

' lille circuit. and will respond to the answering condition imposedupon. the line circuit .from the distantend. by operating. to markingposition.

Thus, the receiving relay lisarranged to control. circuit .equipment which, in well-known manner, may be, arranged to respond in proper sequence to the.trafic signa1s, such as answering and.. busy, from. the called, line. It` mayiurtherf. more vbe arranged. toi function inresponse.to dial.U

pulses from they distant station',y when. the. call..

is in the opposite. direction, for control .of .the selecting operation of telephoneswitches atthe exchange with, which the. repeater is immediately associat.ed.

It will, therefore,. be seen that "in, a .sense the. repeater operates under full-duplex` conditions, inasmuch as it willoperate with either of. two polarities. applied. to either .or .both ends.. of. theV line circuit. However, though tha-repeater is .re-

vquired,tooperatelin both directionsitwill oper.-

ate only in one directi0n at a. time, so .that .forany particular `line condition the repeater. willbe. operatingon a half-' duplexbasis.

This particular condition permits. of. the; introduction. of a feature `of the invention whichv basically resides in renderingthe .receiving relay signals andrestoring the receiving .-relay. circuit to eectiveness upon termination ofthe retransmission of outgoing signals.

In accordancewith a more specicieatureiof the invention,.the controlcircuit from the contactsoi the receivingrelay is `rendered immune,v to operations by the receiving relay during transmission. of outgoing pulses andfor this purposev by-pass1circuitsare provdedfor the receiving. relay contactsior ,temporarily completingthe control` circuit irrespectively of fortuitous operations .bythe receiving relay during 1 transmission` of youtgoing pulses.;

In accordance with' another specific feature of., the invention the by-passis; applied; across, th.,75,.

marking contacts or the spacing contacts of the relay in accordance with the position of the receiving relay at any time, as determined by the line potential applied at the distant end.

The invention will now be described more in detail as to its organization and operation and as applied to a specific embodiment. In the de'- scription reference will be made to the attached drawing, inxwhich:

Figs. 1, 2 and "3^ are diagrammaticrepresentations of diierent typical pulses that may be transmitted in either direction overjthe line circuit; and

Fig. 4 is a circuit diagram of a repeater cir- #cuit represen-tingja1specific application of the invention andiingluding the features of the invention.y

Referringjftocthedrawing, it will be assumed that' a subscribers station is connected to a telephone exchange and desires to make a call to a distant .-subscriberlsgstationg attanother telephone. exchange. nected together overtoll. lines.. y

In accordance. ,with4 the. establishedI .traina conditionsin the .telephone pIant, .the subscriber first7^ removes. hisreceiver. from the hook,` thereby establishing a..calling. condition.. Thereafter the.V subscriber operates. his..ca1lingf. .da1 lwhichtransmits. the Iusualgroups .ofdial-ingpulses... The` dis.- ,tant exchange thensapplies an .answering condi tion to the line circuit andiringsthe calledlsubg: scriber, who. yanswers by .lliiting his-...receiver oft. the hook; ,thereby establishing abusy condition. If the called-.subscribenshould '.b'e lousy, :a .continuous series ,o-ibusy. .pulses.-will`. be .transmitted 1 backito .the calling station at', the rate of80 .pulses permiriute.= Other busy conditions, as forilocalf.v trunks or distanttrnnksiare indicated by similar.. pulses vof different frequenciesi., Upon lvthedistant subscriber disconnecting, prematurely... a lrering AThereafter the markingl current `i`s..,interru10ted bythe dialingpulses.;5 whereupon. vthe .line Ycurrent remainssteady. In the` case,.of.fa subsequent re-v ring. the line -currentis. momentarilyz interrupted.`

Fig. ,2` ".refers. .t`o....the `conditions at. ,the .called end of `the l toll'linawhichnormally is., applying a spacing condition- .to .the ..1ine.. fcircuitf Upon.. answeringthe... called.. exchange. transmits.. a steady. .line current 'which-may, subsequently .be

interrupted by. switchhoolashes.

Fig. ..3 shows conditions.A atthe ,calledlexch'ange when a busy., condition.. exists.. inwhich` case the steady marking. .condition .islinte'rruptedllcy .spac.- ing conditions.

Referring no.w particularly,y to Fig.. 4; the .cir-.- Icuitis that of 'a repeater' stationglflconnehted over; a' tolli line; ,circuiti ILitd ,a -distant "exchange station, represented'at. The .repeaterstaton may-be assumedto ,be l'ocatedatA "an, exchange. to which a pluralityiof.subscribers arelconnectedgl y one ofWhich'is shown'atl Sl The subscriber.. station. Sjnay Vbe a -convention station with a switchliook SH and dial DI. ..The. station I isl .-connectedover .assubscriberfs lirie SII' to.. the repeater .station-. However, ,f`o'r' .the puri-n poses .ofithe. inventiointhe liner circuitt ,Shmayljg 'I'heL two exchanges areconf be; connected to a, toll operatorsl circuit instead` of to a subscribers station, in `which case the `switchhook `may be replacedby a key and the i dial by an operators pulse sending device.`

. The repeater station R comprises the pole changer or sending relay 440|) connected in the line 'circuit SL to receive pulses and `having contacts for alternately applying ground `andbattery to the line circuit TL in sending the pulsesz outgoing from the exchange. i A source: of busy signali4|0 `may be connected through busy key 4|| or `any otherautomatic means for applying the busy signal Vto the circuitwSL foroperation` of relay 460,. The pulse circuit SL through re-` lay .,406 extends `over resistance 443 to negative l fectthe relay`450, lsince proportionate currents'` pass through the two windings in` opposing rei lation in accordance `with the balance between the line, circuit TL and the network BN. l

, The distant exchange E may include a suitable responsive switch control fcircuit 49|. The

station E` may also`1include au repeater circuit similar to the .circuit R` with a receiving relay similar` to relay 450 and with transmitting con tacts 49B similar to those of relay4llll.` i

When the circuit is in the condition shown in the drawing, groundis applied to both ends of the lineiTLand `to the balancing network` `BN` so'` that no current willbe flowing. When relay 400 operatesfbatterytis connected to the bridgeypoint` l? and current flows through one winding over the line circuit to ground at E and through the other winding through `the balancing network to ground. Thewindings being opposed under this condition, relay 456 `would not respond provided' the balance is good. With ,i

ance for charging and discharging thecondenser 442 as the pulse currentrises and declines, respectively, through the resistance 441-11; the charg-` ing and discharging currents momentarily operate relay 44|. The contacts of relay 44| are ineluded in a control circuit from the contacts of the receiving relay 450, through a pair of relays 46| `and 462 for control of a circuit equipment 410 which is part of the exchange equipment and serves to selectively control switches for setting` i up a connection in responseto pulses incoming from a distant exchange to the repeater R. `The equipment 41D will have certain portions, represented by relay 41|, responsive to relay 45|] in spacing position and otherportions, represented by relay 412, responsive to relay 450 in marking position. Relay 46| will be operated `together withrelay 41| overa break `contact on relay 462 and the spacing contact of relay 456; relay 462 will be operated in series with relay 412 over a breakcontact of relay 46| and the marking contact of relay 450.

i The operation `of the circuit will now be de: scribed. `It will be assumed `that the` circuit isin rest condition, as shown in the drawing.

When the subscriber removes the receiver from the hook SH, ground is extended over the subscribers line SL, through the winding of relay 400 and resistance 443. teryto the duplex circuit and over the lineto the distant exchangell` for operation of the'switch control circuit 49|.

The rise `of current through resistance 443 causes a charging current to flow fromthe upper lpositive side of the resistance through branch a` of therectiiier bridge RB,-relay 44|, branch c of the bridge, through condenser 442, tothe lower relay 400 applying ground to the duplex circuit and contacts 49|) at the distant end of the line operated to apply battery, the incoming line current will pass through the upper operating winding to ground at relay 400 and in a direction to overcome the biasing winding and operate relay 45D to marking. If now relay 400 be operated to` also apply battery to `theduple'x circuit, practically no current will flow,` in the line and through the upper` operating winding; the battery at relay 400 will, however,A send current through the lower operating winding on relay 45|! ina direction to keeprelay 45D operated to marking. Thus in receiving incoming, marking,

. and `spacing pulses, the two operating windings of relay 450 will be in cooperative relation to operatethe relay to marking orvspacing respectively. v

.Although the receiving relay 450 is shown in the drawing and is described herein as beine,r connected in a diierentialduplex circuit, it should be understood that in `accordance with the invention, this relay maybe connectedin a duplex c bridge circuit with similar advantages. i

A full-wave rectiiier bridge RB, `relay 44| and `condenser 442 are connected acrosswthe resistance negative side of resistance lll-3. impulse operates relay 44| ground `to a locking circuit for relay llltover` a make contact of the relay and through relay-41| to battery, so that relays 46| and 41| will remain i operated independently of any kick-off by re-` lay 450 due to unbalance currents between the line circuit TL and the balancing network BN` during the t outgoing space-to-mark transition. It will be noted that the operated relay 46| at its break contact keeps open the operating circuit from the marking contact of relay 450 through the relays 462 and 412, therebypreventing false operation of these relays in case of kick-off."

i After the transition has passed into the line cir- 443 `to receive potential` `drops from the resistl` cuit and relay 44| has released, the equipment 410 will again be under control of relay 450.

The subscriber now operates the dial, the contacts of which produce spacingpulsesasshown in Fig. 1, by short interruptions of the ground connection to the line SL. In response tothe first interruption by the dial relay 400 releases. .The decaying `current through resistance 443 now causesthe `condenser 442 to discharge over a cir` .cuit` from the upper positive side of the `condenser, through the branch b of the rectiiier bridge` RB, through relay 44|, branch d and resistance 4,43back to the negative side of condenser 442. This discharging impulse again operates relay 44| for bridging `the spacing contactsof relay 456 by applyingground to the holding` circuit for relays 46|' and 41|. These relays thus will remain operatedfindependently of any kick-oir, of relay 45|) during the mark-to-space *transition in the line circuit. The circuit from the spacing contact of relay 456 also remains open at the break contactof relay 46|; thereby preventing operation of relays 462 and 412. After Relay 46B `applies bat- The` charging which connects' and 41|, as described ab-ove.v During each pulse series relay 464 will repeat each marking and spacing transition and relay 44| performs i'ts locking operation during each transition. Dur-A ing an interval between pulse series relay 40|! re'- mains attracted. f

After the dialing has been completed steady battery will be applied to the duplex circuit by dialing relay 464 in marking position.

When the answering condition. is established. at the distant end ofthe line TL, as bythe contacts 490 operating to battery, relay 45t will operate to marking, thereby applying ground to the equipment 416 and particularly to relay 412 for the proper reception ofthe answering signal. Relay 450 also opens the control circuit for relays 46| and 41|, thereby releasing relay 46| which in turn completes the control circuit from the marking contact of relay 456 through relays 462 and 412. Relay 462, on operating, opens'the control circuit fro-m the spacing contact of relay 456 to relays 46| and 41| and at its make contact it prepares the lockingA circuit from relay 44| through relays 452 and 412.

vThe desired connection may now be assumed to have been established and the called subscriber will be rung from theringing equipment at the exchange E. When the called subscriber answers, the conversation may be carriedonover a talking circuit, not shown in the drawing, but which would be closely associated with the toll line TL, as by being included in the same cable quad or by some composite arrangement.

It will now be assumed that some special circumstance arises, asv for example, that the distant subscriber hangs up prematurely, necessitating the sending of further pulses from the calling end, for example, a rering pulse, as indicated in Fig. 1. Relay 466 would momentarily interrupt the battery connection to the toll line and relay 44| will be operated once for each transition. Under this condition relay' 44| closes the locking circuit for relays 462 and 412 by bridging the marking contacts of relay 456. The

control' circuit from the spacing contact of relay through the windings of relay 45o and network BN to ground, causing the relay to operate in unison with the busy pulses and applying correspending busy pulses to the circuit 410 for suitable response. The effect of this upon the switch control circuit will depend upon the operating characteristics of relays 46| and 462.

Thus, if these two relays are made slow to release to such an extent that they' will remain operated during each busy pulse, the functions would be as follows: With relay 466 in marking position and relay 462 operated, the interrup tions at the marking Contact of relay 456 will cause relay' 412 to' oscillate forl the proper busy control, whereas relay 462 will remain operated.

Thos, re1ays 46| and 41| win remainy unaffected by the busy' impulses. However, if relays 46| 'and 462 should be fast ,enough to follow the operas tions of relay 456 dur-ing a busy signal., the funer tions would be as follows: Again assuming that relay 456 is' in marking position when the busy proper busy control in response to the space-toe mark and mark-to-space transitions, respectively, of the incoming busy signals.

In the case of a call incoming to repeater sta'-Y tion R, relay 45D will be operated to marking in responsek to the application of battery at the disi tant exchange E to the line. ruptions', as by dial pulses, would cause relay 450 to oscillate and apply corresponding pulses.

to the circuit 410 for selective operation of switches. The control of the circuit 410 will again depend upon the operating characteristics of relays 46| and 462, as described above in the case of busy signals. However, these relays may be made slow enough to hold over the comparatively short dial pulses and yet be fast enough to respond to each transition ofthe slower busy pulses. Inl response to this selective operation a switch may ultimately connect the subscribers station S' to thev repeater station R. This switch` is diagrammatically representedv by contacts LF; contacts LF may also represent a line nder switch for connecting the subscribers station to the repeater in response to a call by the sube scriber. With the subscriber connected in this manner, switchhook flashes may be transmitted back tor station E in the manner' already described.

In case the subscriberS is busy, the connection at switch` LF will remain open and the circuit 416 may be arranged to automatically operate the key 4|! for application of busysignal frominterrupter 4|ll. This will cause` relays 40|) and 44| to operate in unison with the busy pulses and establish locking circuits. for relays 46|, 41| and 462, 412, as already described;

On observing the busy signal and distant calle ing station at E will hang` up and thus will 'send' a disconnect signal by applyingste'ady ground over contacts 496, through the windings of relay 455i, thereby loperating the relay back to' spacing. Thev circuit 416 responding to this operation may be arranged to automatically disconnect the ine terrupter 4|!) by means of contacts 4H, where-i upon t'he whole system will be restored to idle condition.

The inductance' I is inserted between the cone tac'ts of relay 466and the apex` P for thel 'pur' pose of shaping theoutgoing pulses in anydesired manner.

From the description presented above, itwill be understood that the purpose of relays 44|', 46| and 462 is attained during transmission of outgoing pulses. Thus, in accordance with the in vention and asI already explained' above, the' bald ancing network is of simple construction and is designed so that networks 'of this kind may be usedv universally with, toll linesv of differentrlengtlis'v and different characteristics. Y ItV is',y of 4course,l

Subsequent inter-4 standardized types `of network may be provided, i

each type to be used with lines having characterfistics within comparatively narrowlimits and" the different types of networks supplementing one another to satisfy the requirements of all types of lines. Thus, any one network may be connected to a particular line circuit and establish only` an approximate balance; and the unbalance may be soA pronounced that the receiving relay ordinarily would kick-off transmission of outgoing pulses.

during It is furthermore well knownthat long line` circuits, as used `in toll serviceare` subject to sporadic variations in their characteristics which` are beyond control.A Where a very close balance` 'is necessary it has been the practice'to continui ously observe the line conditions and correspondinglyvary the balanceby the network.

In the protecting `equiprnentoshown in the drawing the contacts ofthe receiving relay are" disabled during the sending of pulsesgthereby permitting the relay 450 to fkick-of without disturbing eiect upon the equipment 410. Thus, appreciable leeway is aorded in the balancing of fthis circuit, therebypermitting the use of` universal networks for approximate balancing over wide ranges `and also in special cases obviating theneed of continual readjustment of the l network.` l

By switching the` protecting, circuit in accordance with the position of the receiving relay it connected `to be responsive to incoming line currents for the conditioning of said auxiliary circuit'means for the proper prevention of said control under both ofsaid steady state conditions,

said" auxiliary circuit means comprising locking e circuit means including a multiple path foreach i of said contacts `for locking said receiving circuit'` in marking or spacing condition irrespectively of false operation of said receiving relay means, and said switching `relay `means having contacts for completing one or the other of said multiple paths in accordance with said steady state conditions.

3. A two-way signaling system comprising `an approximately balanced `line circuit normally maintained alternately in markingand spacing steady state conditions `from the remote end, receiving relayfmeans at `the other end of said line circuit having contacts for control of a receiving circuit in response to incoming marking and spacing signal pulses superimposed `on said steady state conditions, auxiliary "circuit means connected to be responsive to outgoing marking and prising `locking circuit means connected to said is insured that fortuitous operations `by thisrelay from either of itspositions will have no disturbing effect. i

i It should be understood that the invention may readily be adapted for application to a telegraph repeating system with similar advantages, where sending takes place" against two different` signaling conditions imposed What is claimed is:

by the `-remote station.'

1`. A two-way signaling systemcomprisingan approximately balanced line circuit` normally maintained alternately in marking and spacing steady state conditions, receiving relay means j havingcontactsfor control of a receiving circuit e means for the proper prevention of said control said re1ay means contacts in response to unbalance currents i due to` said outgoing pulses, and switching `means connected to be responsive to said steady state conditions for the conditioning of said auxiliary circuit means for the proper prevention `of said control under both of said steady state conditions, said auxiliary circuit means comcontacts for locking said vreceiving circuit in marking or spacing conditionin accordance with said alternate steady state conditions, said switching means comprising relay means for conditioning said locking circuit means `in accordance with said steady state conditions, and said auxiliary circuit means further comprising pulse relay means connected to` be responsive to outgoing signal transitions for producing a locking pulse through said locking circuit means during a false l operation of said receiving relay means.

4. A two-way signaling systemA comprising an `approximately balanced line circuit normally maintained alternately in marking and spacing steady state conditions from' the remote end, rei ceivinglrelay means having contacts for control of a` receiving circuit in response to incoming l marking and spacing signal pulses superimposed l `on said steady state conditionsauxiliary circuit means connected to be responsive to `outgoing i marking and spacing signal pulses to prevent said under both of `said steady statel conditions, said auxiliary circuit `means comprising locking circuit means connected to said contacts for locking l said receiving circuit in marking or spacing" conmaintained 'alternately in marking and spacing l steady state conditions, receiving relay means i having contacts `for control of a receiving circuit in response to incoming `marking and spacing signal pulses superimposed on said steady state conditions, auxiliary circuitmeans connected to be responsive to outgoing marking and spacing signal pulses vto prevent said control by said con# tacts in response to `unbalance currents due to said outgoing pulses, and switching relay means H respectively, for locking said receiving circuit.`

control by said contacts in response to unbalance currents due to said outgoing pulses, and switch- .ing relay means connected to be responsive to said steady state conditions for conditioning said `auxiliary circuit means for theproper prevention of said control under both of said steady `state conditions, said auxiliary circuit means comprisinglocking circuit means connected to said contacts for locking said receiving circuit inmarking or spacing condition in accordance Withsaid alternate steady state conditions and pulse relay :meansconnected to be responsive to outgoingsigl,

nal transitions for producing locking pulses through` said locking.circuit means during false operations of said receiving relay means, and said switching relay means comprising a marking relay and a spacing relay connectedto said locking circuit means one at a time to be locked by said lockingpulses in the marking or spacing conditions, 

